I. Field of the Invention
This invention relates generally to timing analysis and, more particularly, to detection of timing errors in generation of signals.
II. Description of the Related Art
In cellular telephone systems, personal communications systems and wireless local loop systems using a code division multiple access (CDMA) coding technique, remote units (typically cellular telephones) use a common frequency band for communication with all base stations in the system. Use of a common frequency band adds flexibility and provides many advantages including the simultaneous reception of communication signals exchanged between sets of remote units and base stations. An over-the-air interface for implementing a CDMA base cellular telephone system is defined in the IS-95 standard promulgated by the Telecommunications Industry Association (TIA), as well as other well known standards bodies. Additionally, a cellular telephone system configured substantially in accordance with the use of IS-95 is described in U.S. Pat. No. 5,103,459 entitled "System and Method for Generating Signal Waveforms in a CDMA Cellular Telephone System" assigned to the assignee of the present invention and incorporated herein by reference.
In a typical CDMA communications systems, both the remote units and the base stations discriminate the simultaneously received signals from one another via modulation and demodulation of the transmitted data with high frequency pseudo-noise (PN) codes, orthogonal Walsh codes, or both. For example, IS-95 separates the set of transmission from the same base station by the use of different Walsh codes for each transmission, while the transmissions from different base stations are distinguished by the use of a uniquely offset PN code.
In order for a communication to be conducted properly in a CDMA system the state of the particular codes selected must be synchronized at the transmit and receive systems. Synchronization is achieved when the state of the codes at the receive system are the same as those in the transmit system, less some offset to account for any processing and transmission delay. In an IS-95 compliant CDMA system, such synchronization is facilitated by the transmission of a pilot channel from each base station comprised of the repeated transmission of the uniquely offset PN code (pilot PN code). In addition to facilitating synchronization, the pilot channel allows identification of each base station relative the other base stations located around it using the pilot channel phase offset.
To synchronize with the transmission from a base station, a remote unit performs repeated time offset searches with the pilot PN code until the pilot channel is detected. The remote unit is equipped with a searching element for performing such a search that also allows the remote unit to track the signal strength of the pilot signal from a group of base stations including the neighboring base stations. Further information on searching processes can be found in co-pending U.S. patent application Ser. No. 08/316,177 entitled "MULTIPATH SEARCH PROCESSOR FOR A SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM" which is assigned to the assignee of the present invention and incorporated herein. Once a first pilot channel is detected, the detection of other pilot channels is made easier by continuing to search at the various known offsets relative to the detected pilot channel. This searching may further be facilitated by transmission of suggested search offsets from the base station to the remote unit based on the known configuration of the cellular telephone system. The searching element in the remote unit begins its search at the suggested offset or offsets, and therefore is more likely to detect the pilot channel that if the search was initiated at some random start point.
Remaining synchronized with each base station when a telephone call or other communication is in progress is especially useful as it assists in conducting a soft handoff rapidly. When a soft handoff is established the remote unit is called on to begin processing transmissions from two or more base stations simultaneously. U.S. Pat. No. 5,267,261 entitled "Mobile Station Assisted Soft Handoff in a CDMA Cellular Communications System," which is assigned to the assignee of the present invention and which is incorporated herein, discloses a method and system for providing communication with the remote unit through more than one base station during the handoff process. Soft handoff can be contrasted to hard handoff where the interface with a first base station is terminated before the interface with the second base station is established. Additionally, transmit power control with a particular base station can only commence once the soft handoff has been established with that base station, further necessitating the soft handoff be conducted rapidly.
Inherent in the searching process of a remote unit is the need for the base station timing to be precisely aligned with respect to the other base stations in the system and for the base station timing to remain precisely aligned during the searching process. Either a drifting time or a constant absolute error in the time may cause the searching mechanism to degrade or fail. If the searching mechanism fails, soft handoff may not be established. It should also be noted that if soft handoff is not established, power control does not operate properly. If power control does not operate properly, system performance and capacity fall. One or more systems for base station and remote unit power control are disclosed in U.S. Pat. Nos. 5,056,109, 5,265,119, 5,257,283 and 5,267,262 which are incorporated herein.
Therefore, base station timing is extremely important to system operation. To illustrate the importance of base station timing, IS-95 states that each base station shall transmit its pilot signal at the given time offset with an error that should be less than 3 microseconds and an error that shall be less than 10 microseconds in order to establish uniformity throughout the industry. This language means that, although 10 microseconds of offset error may be tolerated, the association highly recommends having an error of less than 3 microseconds. Base stations are typically equipped with universal time sources such as global positioning satellite (GPS) receivers in order to achieve the precision required by IS-95. The universal time output from a GPS receiver is input to the base station which uses the universal time to align, among other things, its pilot signal to the proper offset.
Despite the restrictive time requirements, a CDMA system is sufficiently robust that it may continue to operate when a failure in the GPS system or other timing mechanism introduces a poorly synchronized base station in to the system. Such a failure would result in system performance degradation, however, and therefore would be highly undesirable. The lack of complete system failure makes detection of an unsynchronized base station difficult, even though the overall performance and capacity of the cellular telephone system may be impacted substantially. Therefore, it is advantageous to have a means by which base station timing may be tested.
However, testing base station timing is extremely difficult and imprecise using conventional methods. Conventional test methods of analyzing base station timing have consisted of connecting a conventional digital timing analyzer to test points at various locations throughout signal flow within the base station. This method is imprecise because error may be introduced in the base station timing at later points in the base station signal flow which would not be detected.
FIG. 1 shows a typical existing base station 10. The base station 10 is comprised of a global positioning satellite (GPS) receiver 20, a digital processing unit 30, an RF and analog processing unit 40, and power amplifier (PA) 50. Any method of acquiring timing may be used such as triangulation or cesium standard devices. In the exemplary embodiment of FIG. 1, the GPS receiver 20 is a commercially available unit. When initially powered on, the GPS receiver 20 enters a navigation mode in which it monitors signals from four different satellites to determine its exact location on the earth. Once it has determined its own location, the GPS receiver 20 enters timing mode and monitors a signal from a single satellite to determine `absolutely`the time of day on a continuous basis.
The GPS receiver 20 provides a timing signal to a digital processing unit 30 and a radio frequency (RF) and analog processing unit 40. The digital processing unit 30 uses the timing signal to create a pilot signal and for various other functions. For reasons described below, the absolute time as received from the GPS receiver 20 must be adjusted by the base station to accommodate delays within the base station. For example, the GPS receiver can have signals bias and errors due to multipath transmissions. The adjustment is made in a base station timing adjust unit 32 which essentially advances the absolute time value received from the GPS receiver 20 by a fixed amount of time to account for delay introduced by the base station 10 as a whole so that the signal generated at an antenna 60 represents the correct time as closely as possible.
The output of the base station time adjust unit 32 is used, among other uses, to create a pilot signal in a pilot signal generation unit 34. The pilot signal generation unit 34 creates a digital representation of the signal which shall be transmitted. The digital signal is passed to the RF and analog processing unit 40.
Within the RF and analog processing unit 40, the digital signal is converted to an analog signal using a digital to analog converter 42. The resulting analog signal is subject to several processing actions within the RF and analog processing unit 40 including filtering and upconversion. The analog signal is passed through a standing acoustic wave (SAW) filter 44 and, then, to a power amplifier 50 and the antenna 60 for transmission over the wireless link. Each process within the digital processing unit 30, the RF and analog processing unit 40, the power amplifier 50 and the antenna 60 introduces some delay.
One of the main sources of delay within the base station 10 is the SAW filter 44. A SAW filter is ideal for use with wideband digital signals. SAW filters provide flat group delay, flat passband response and extreme rejection in the stopband. However, a SAW filter inherently introduces a rather substantial flat delay. For example, a typical SAW filter used in a cellular base station may introduce 20 microseconds of delay. Obviously, the delay of the SAW filter 44 would itself cause the non-conformance with the IS-95 standard unless some correction were made.
The correction is made in the base station time adjust unit 32. At initial deployment, an end to end delay measurement is taken of the entire assembled base station. The base station time adjust unit 32 provides an artificial advancement of timing equal to the end-to-end delay measurement taken.
Even if the base station time adjust unit 32 initially compensates perfectly for the delay introduced by the base station 10, over time the timing may change. For example, the power amplifier 50 and the antenna 60 are typically exposed to environmental temperature changes while the remainder of the base station 10 is likely to be housed in a temperature controlled environment. If the delay characteristic of the power amplifier 50 and the antenna 60 change over temperature, the timing of the resultant output signal also changes with temperature. Also, it is possible for the GPS receiver 20 to malfunction and to drift in time. In addition, a phase locked loop within the RF and analog processing unit 40 may become unlocked and drift in phase introducing a continuous shift in the output time. Several modes of failure within the digital processing unit 30 may also introduce absolute or drifting timing errors such as ringing clock signals.
When a small absolute or drifting error occurs in one base station, the overall system performance of the cellular system begins to degrade. However, the failure is not typically catastrophic or easily detectable. For example, if an undetected error in base station timing causes a delayed entry into soft handoff, an unusually high error rate may be observed at neighboring base stations only when a remote unit is located within a soft handoff region of the coverage area of the errant base station.
Also note that a measurement of timing made in the base station at any other point in the process other than after the antenna 60 may not be an actual reflection of the timing of the signal produced by the base station 10.
FIG. 1 shows the base station 10 in one of the most basic environments in which it may operate. In other more elaborate configurations, many other elements may be present which may introduce timing error into the system. For example, the base station 10 may be connected to a cable television system. In such a configuration, optical elements are used to convey the signal from the base station to the coaxial television cable. Along the length of the cable, a series of radio antenna devices (RADs) couple signals to the wireless link. In such a configuration, the delay introduced becomes even more volatile and hard to measure. A similar situation may be found if optical fibers or microwave beams are used to transmit signals to remote antennas. In these more elaborate configurations, the method and apparatus of the present invention of testing base station timing from within the base station coverage area become even more essential.
The most comprehensive measure of base station timing is achieved by measuring the transmit RF signal after the signal has left the antenna. However, until the advent of the present invention, such measurements were not possible. Therefore, there has been a long felt need of the industry to have a comprehensive and precise method of measuring base station timing.